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LMV721 Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LMV721 Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 15 page Application Notes (Continued) 4.0 TYPICAL SINGLE-SUPPLY APPLICATION CIRCUITS 4.1 Difference Amplifier The difference amplifier allows the subtraction of two volt- ages or, as a special case, the cancellation of a signal common to two inputs. It is useful as a computational ampli- fier, in making a differential to single-ended conversion or in rejecting a common mode signal. 4.2 Instrumentation Circuits The input impendance of the previous difference amplifier is set by the resistor R 1,R2,R3 and R4. To eliminate the problems of low input impendance, one way is to use a voltage follower ahead of each input as shown in the follow- ing two instrumentation amplifiers. 4.2.1 Three-op-amp Instrumentation Amplifier The LMV721/722 can be used to build a three-op-amp in- strumentation amplifier as shown in Figure 6 The first stage of this instrumentation amplifier is a differential-input, differential-output amplifier, with two volt- age followers. These two voltage followers assure that the input impedance is over 100M Ω. The gain of this instrumen- tation amplifier is set by the ratio of R 2/R1.R3 should equal R 1 and R4 equal R2. Matching of R3 to R1 and R4 to R2 affects the CMRR. For good CMRR over temperature, low drift resistors should be used. Making R 4 slightly smaller than R 2 and adding a trim pot equal to twice the difference between R 2 and R4 will allow the CMRR to be adjusted for optimum. 4.2.2 Two-op-amp Instrumentation Amplifier A two-op-amp instrumentation amplifier can also be used to make a high-input impedance DC differential amplifier (Fig- ure 7). As in the two-op-amp circuit, this instrumentation amplifier requires precise resistor matching for good CMRR. R 4 should equal to R1 and R3 should equal R2. 4.3 Single-Supply Inverting Amplifier There may be cases where the input signal going into the amplifier is negative. Because the amplifier is operating in single supply voltage, a voltage divider using R 3 and R4 is implemented to bias the amplifier so the input signal is within the input common-common voltage range of the amplifier. 10092220 FIGURE 4. Cancelling the Error Caused by Input Bias Current 10092221 FIGURE 5. Difference Application 10092230 FIGURE 6. Three-op-amp Instrumentation Amplifier 10092222 FIGURE 7. Two-op-amp Instrumentation Amplifier www.national.com 10 |
Similar Part No. - LMV721_06 |
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Similar Description - LMV721_06 |
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